(2)Possible, but I can still imagine large civilizations of people whose utility function is weighted such that "99.9999% death plus 0.0001% superman" is inferior to "continued mortal existence."

You have to keep in mind that subjective experience will be 100% superman. The whole idea is that the MWI is true and completely convincingly demonstrated by other means as well. It is like if someone would tell you: you enter this room and all you will experience is that you leave the room with one billion dollars. I think it is a seducing prospect.

Yet another analogue: Assume that you have the choice between the following two scenarios:

1) You get replicated million times and all the copies will lead an existence in hopeless poverty

2) You continue your current existence as a single copy but in luxury

The absolute reference frame may be different but the relative difference between the two outcomes is very similar to those of the above alternative.

Possible additional motivation could be given by knowing that if you don't do that and wait a very very long time, the cumulative risk that you experience some other civilization going superman and obliterating you will raise above a certain threshold. For single civilizations the chance of experiencing it would be negligible but for a universe filled with aspiring civilizations, the chance of experiencing at least one of them going omega could become a significant risk after a while.